The present application relates to organic semiconductor materials, organic semiconductor thin films and organic semiconductor devices.
Semiconductor devices using organic semiconductor materials are able to decrease a manufacturing cost as compared with related-art semiconductor devices using inorganic semiconductor materials such as Si (silicon) and they can be also expected as semiconductor devices with flexibility as well. Then, various kinds of organic semiconductor materials such as polythiophene and rubrene are now under studies as the organic semiconductor materials, and it has been reported that a transistor including a channel forming region made of these organic semiconductor materials may have mobility of substantially the same as that of a transistor including a channel forming region made of amorphous silicon (see APL Vol. 80, No. 6, 1088-1090 (2002), for example).
When the channel forming region is made by these organic semiconductor materials, since these organic semiconductor materials are difficult to be dissolved into an organic solvent and the application of a coating process to these organic semiconductor materials is difficult and it is customary that semiconductor films are exclusively formed by a vacuum evaporation coating method. On the other hand, a simple alkyl chain and other substituents are introduced into these organic semiconductor materials to cause an organic solvent to have affinity so that these organic semiconductor materials can be dissolved into the organic solvent. In actual practice, poly-3-hexylthiophene (P3HT) can be dissolved into an organic solvent such as chloroform and toluene and it has been reported that a channel forming region could be formed by a coating process such as a spin coating method (see APL 69 (26), 4108-4110 (1996) for example).
On the other hand, a polyacene compound, which is a condensed polycyclic compound, is a molecule having a π electron conjugated system similarly to polyacetylene and polyphenylene. In addition, the polyacene compound has a small bandgap as compared with polyacetylene and the like from theoretically and it is a compound which can be expected to have excellent functions as an organic semiconductor material. Substituents that have been introduced into the polyacene compound can be used so that it can be coupled to their molecules and a functional group on the surface of an insulating film. Also, these substituents can be used to control a distance, a position and an arrangement of an acene bone and patterning and the like. The polyacene compound is a compound in which benzene rings are coupled in a straight line fashion. A polyacene compound without substituents has properties in which it becomes difficult to be dissolved into an organic solvent in accordance with the increase of the number of the benzene rings. In particular, a polyacene compound greater than pentacene having five benzene rings coupled loses its solubility to almost all of organic solvents and it is very difficult to form a uniform film based on a suitable method such as the spin coating method. Even if a uniform film can be formed by using such polyacene compound based on the spin coating method and the like, it is unavoidable that organic solvents and temperature conditions available in the spin coating method and the like will be extremely limited (for example, trichlorobenzene and 60 to 180° C.). Also, it is widely known that stability of the polyacene compound is lowered as the number of benzene rings is increased and that pentacene is oxidized by oxygen in the air. That is, pentacene is poor in oxidation resistance.
2,3,9,10-tetramethylpentacene was reported as an example in which substituents are introduced into a polyacene compound (see Wudl and Bao, Adv. Mater Vol. 15, No. 3 (1090-1093), 2003). However, this 2,3,9,10-tetramethylpentacene can be slightly dissolved into warmed 1,2-dichlorobenzene, and hence a channel forming region constructing an FET (field-effect transistor) is formed by a vacuum evaporation coating method.
Also, Japanese Published Patent Application No. 2004-256532 has described that 2,3,9,10-tetramethylpentacene and 2,3-dimethylpentacene are dissolved into 1,2-dichlorobenzene. However, they can be dissolved into 1,2-dichlorobenzene at 120° C. but the fact that they are dissolved into 1,2-dichlorobenzene at room temperature is not described in the above Japanese Published Patent Application 2004-256532.
J. Am. Chem. Soc. 124, 8812-8813 (2002) has reported a technology in which a substituent, which is capable of carrying out thermal reversible reaction, is introduced into pentacene to prepare a solution in the pentacene precursor state with high solubility relative to an organic solvent, this solution is coated on the substrate and heated, thereby resulting in a pentacene thin film being formed on the substrate.
Also, compounds in which substituents are introduced into pentacene are known from a long ago and D. R. Maulding et al. has reported syntheses of several pentacene derivatives in Journal of Organic Chemistry, Vol. 34, No. 6, 1734-1736 (1969). Also, in recent years, Takahashi et al. and Anthony et al. have reported many pentacene derivatives. For example, refer to Organic Letters Vol. 6, No. 19, 3325-3328 (2004) and Organic Letters Vol. 4, No. 1, 15-18 (2002).
As described above, although the polyacene compound is a compound which can be expected to exhibit excellent functions as an organic semiconductor material, the polyacene compound is difficult to be dissolved at low temperature (for example, room temperature) and which is therefore not suitable for use with a coating process such as a spin coating method.